Techniques and equipment for ion beam measurement useful in controlling various aspects of the beam have been developing over some time. Among the more commonly used equipment are grid harps and Faraday cups. Each has its own unique construction, and both are used in a variety of applications, principally in connection with testing or manufacturing methods.
A grid harp is a device for measuring intensity distribution of an accelerated ion or electron beam principally in two transverse directions. A grid harp typically includes a frame member having an opening of orthogonal dimensions x and y, through which the beam passes and across which is disposed a grid of wires consisting of a first set of wires in the x direction and a second set of wires in the y direction. Wires used for the x and y planes collect charged particles, and the collected charge is then converted to a value representing an intensity level. The array of wires then produces a thin cross-sectional mapping of the intensity level at various points in the beam. By using a greater number of wires, more precise measurements can be obtained. For particles having a range which is greater than the thickness of harp wires themselves, secondary electrons can be measured.
However, grid harps tend to be very expensive, require extensive modifications of the accelerator beam line, and measurements using grid harps tend to be time consuming and often yield incomplete data. Further, measurement systems using grid harps also require a compressed air actuated, high vacuum feedthrough, and complex processing electronics.
A Faraday cup is a detector that measures, at a fixed location, the current in a beam of charged particles. Faraday cups are typically used in arrays for various applications, and have the advantage of being robust and able to measure an ion or electron stream absolutely. In its simplest form, a Faraday detector would consist of a metal cup or housing mounted on an insulator. The cup would be placed in the path of the particle beam, and an electrical lead is attached which conducts the current to a measuring device.
A Faraday cup is typically connected to an electrometer to measure the current of the charged particles collected by the cup. According to Gauss' Law, the charge collected on the Faraday cup is the induced charge. Faraday cups are highly regarded for accuracy because of the direct relation between the measured current and the number of ions.
When using an ion beam, it is desirable to have the ability to measure properties of the beam, such as its density, its position and orientation, its diameter, etc. Various methods and software for accomplishing this are known in the art, but their use is typically undesirable due to inaccuracies and time consuming procedures. Further, many of these methods rely on processes that produce strictly analytical information which can only be subjectively interpreted.
It would therefore be highly desirable to have a method and apparatus for measuring properties of an ion beam at selected locations that would reliably yield data that could be used as feedback for subsequent control and shaping of the beam to attain continuously desired properties either manually or automatically.
Further it would be highly desirable to have a method and an apparatus that would measure ion beam properties and yield visual information directly correlated to such properties which could be used in the adjustment of various beam attributes.